Do you switch the computer off during this period, i.e. it doesn't get slow if you leave it on overnight?

If it happens with the machine on, try this thread Might be some new virus/spyware thing going on in that case, for all I know

If it's only when the machine is off, then it might be time to think about replacing the battery on the motherboard that the computer uses to keep track of time when it has no mains power